Drill cuttings and solid waste from oil and gas exploration and production wells are often reinjected into previously drilled well bores. For example, Edward Malachosky et al U.S. Pat. No. 4,942,929, issued on Jul. 24, 1990, discloses a method for separating gravel from fluid and cuttings produced during drilling of a well bore. The remaining material, which may contain sand, mud and other materials, is circulated through a centrifugal pump and either reinjected into the same well bore or into a preexisting well bore. This method requires that the gravel components be disposed of, hopefully as a construction material. Also, the method does not provide control of the particle size of the solid component in the slurry mixture injected into the well bore.
In a similar manner, James Jackson U.S. Pat. Nos. 5,109,933 and 5,129,469 issued respectively on May 5 and Jul. 14, 1992, disclose a method in which the drill cuttings are conveyed to a shearing and grinding system. The shearing and grinding system comprises one or more receiving tanks and an associated centrifugal pump that recirculates a mixture of the cuttings and water between the pump and the tank. The centrifugal pump is capable of reducing the solid cuttings to particles having a size of from less than 20 .mu.m and as large as, it has been found, 300 .mu.m. This mixture is then injected, under high pressure, into a subsurface formation by an injection pump. Thus, the size of the solid particles injected into the underground formation may range from less than 20 .mu.m to at least 300 .mu.m.
The above methods of disposing drill cuttings require that the processed slurry be pumped into an annular zone between a wellbore casing and the earth formation, by a high pressure injection pump, at a pressure sufficient to fracture the targeted earth formation. It has been found that liquids containing solid particles plug and seal the pores in sand formations, making it virtually impossible to inject large amounts of the fluid without applying pressures which may fracture the formation. For example, in carrying out the downhole injection of drill cuttings in a recent North Sea development project, a slurry injection pressure of 2,500 psi (17,225 kPa) was required to assure initiation of fractures in the sand formation selected for slurry deposition. Fracturing may allow the wastes to escape into other stratum that could contain usable quality water and hydrocarbons. The application of pressure sufficient to fracture impermeable shales and clays encasing the porous formation is unacceptable for the disposal of large quantities of slurry wastes. Consequently, on-site, high pressure waste disposal through the annulus of casing which fracture the recipient formation is being permitted for the disposal of only relatively small quantities of non-hazardous oil field wastes.
The present invention is directed to overcoming the problems set forth above and to providing a method for advantageously depositing large quantities of oil and gas production operation and other waste materials in underground porous formations. It has been discovered that the ability of underground porous geologic formations to accept slurry mixtures containing solid particles is very dependent upon the size of the solid particles in the slurry. This is particularly true of formations consisting of sand.
Specifically, it has been found that in non-sand porous formations, for example vuglar limestone, if the size of the solid particles in the slurry is reduced to no more than 50 .mu.m, and preferably less than 35 .mu.m, that plugging of pore openings and formation of flow-inhibiting sedimentary deposits within the formation is minimized if not avoided. As a result of controlling the size of solid particles in the slurry mixture, almost no impediment to flow and distribution of the slurry throughout the formation are introduced, and consequently much lower injection pressures, are possible to distribute the mixture throughout the formation. This discovery now makes it possible to greatly increase the utilization of underground porous formations as a result of increased storage capacity and the significantly reduced danger of fracture damage to surrounding formations. Thus underground porous formations that are sealed by surrounding formations are now usable for the disposal of solid waste materials, including radioactive and hazardous materials.
It is therefore desirable to have a method for controllably reducing the particle size of solid materials in a slurry solution to less than a very small predetermined size prior to depositing the slurry, at a relatively low pressure, into an underground porous formation.